Essentials of Anatomy and Physiology

5. Saliva—amylase digests starch to maltose; water
   dissolves food for tasting and moistens food for
   swallowing; lysozyme inhibits the growth of bacte-
   ria (see Tables 16–1 and 16–2).

Pharynx—food passageway from the oral
cavity to the esophagus

1. No digestion takes place.


2. Contraction of pharyngeal muscles is part of swal-
   lowing reflex, regulated by the medulla.

Esophagus—food passageway from pharynx
to stomach

1. No digestion takes place.


2. Lower esophageal sphincter (LES) at junction with
   stomach prevents backup of stomach contents.

Structural Layers of the Alimentary Tube
(see Fig. 16–4)

1. Mucosa (lining)—made of epithelial tissue that
   produces the digestive secretions; lymph nodules
   contain macrophages to phagocytize pathogens
   that penetrate the mucosa; thin layer of smooth
   muscle to ripple the epithelium.


2. Submucosa—areolar connective tissue with blood
   vessels and lymphatic vessels; Meissner’s plexus is a
   nerve network that innervates the mucosa, part of
   the enteric nervous system that extends the entire
   length of the alimentary tube.


3. External muscle layer—typically an inner circular
   layer and an outer longitudinal layer of smooth
   muscle; function is mechanical digestion and peri-
   stalsis; innervated by Auerbach’s plexus, part of the
   enteric nervous system; sympathetic impulses
   decrease motility; parasympathetic impulses
   increase motility.


4. Serosa—outermost layer; above the diaphragm is
   fibrous connective tissue; below the diaphragm is
   the mesentery (serous). The peritoneum (serous)
   lines the abdominal cavity; serous fluid prevents
   friction between the serous layers.

Stomach—in upper left abdominal quadrant;
a muscular sac that extends from the esoph-
agus to the small intestine (see Fig. 16–5)

1. Reservoir for food; begins the digestion of protein.


2. Gastric juice is secreted by gastric pits (see Tables
   16–1 and 16–2).


3. The pyloric sphincter at the junction with the duo-
   denum prevents backup of intestinal contents.

Liver—consists of two lobes in the upper

right and center of the abdominal cavity

(see Figs. 16–1 and 16–6)

1. Functional unit is the hexagonal liver lobule: liver
   cells, sinusoids, branches of the hepatic artery and
   portal vein, and bile ducts.


2. The only digestive secretion is bile; the hepatic
   duct takes bile out of the liver and unites with the
   cystic duct of the gallbladder to form the common
   bile duct to the duodenum.


3. Bile salts emulsify fats, a type of mechanical diges-
   tion (see Table 16–1).


4. Excess cholesterol and bilirubin are excreted by the
   liver into bile.

Gallbladder—on undersurface of right lobe

of liver (see Fig. 16–6)

1. Stores and concentrates bile until needed in the
   duodenum (see Table 16–2).


2. The cystic duct joins the hepatic duct to form the
   common bile duct.

Pancreas—in upper left abdominal quadrant

between the duodenum and the spleen (see

Fig. 16–1)

1. Pancreatic juice is secreted by acini, carried by pan-
   creatic duct to the common bile duct to the duode-
   num (see Fig. 16–7).


2. Enzyme pancreatic juice contains enzymes for the
   digestion of all three food types (see Tables 16–1
   and 16–2).


3. Bicarbonate pancreatic juice neutralizes HCl from
   the stomach in the duodenum.

Small Intestine—coiled within the center of

the abdominal cavity (see Fig. 16–1); extends

from stomach to colon

1. Duodenum—first 10 inches; the common bile duct
   brings in bile and pancreatic juice. Jejunum (8 feet)
   and ileum (11 feet).


2. Enzymes secreted by the intestinal glands complete
   digestion (see Tables 16–1 and 16–2). Surface area
   for absorption is increased by plica circulares, villi,
   and microvilli (see Fig. 16–8); microvilli are the
   brush border.


3. The villi contain capillary networks for the absorp-
   tion of water-soluble nutrients: monosaccharides,

The Digestive System 391